Hey Andrew,
I saw that information alright, but that might just be them I'd say. No other suppliers in Europe are complaining about shortage of balsa.
Wind turbine Blades are not using balsa core for quite a long time now, and replaced with plastic or composite materials. Maybe in China some are still using it, but I would say that will fade.
Regarding cost of balsa, I see in Slek, 2€ for a 3mm sheet, when without looking too hard, I buy it 1.30€. That's 70cents difference, and my sheets are 1000x100mm not that weird 915mm (well, weird to me, must be some imperial things I guess
)
Now, that might be a different story for you guys next year, depending of what they are doing with Brexit, you may not have really a choice if we include import duties etc to your order from Outside of the UK! I'll sneak some for you in the back of my car